Helium dimer

Fig. 1. Counterpoise-corrected and uncorrected for the helium dimer. The dashed lines represent the estimated CBS limit.

A particularly interesting group of reactions that fall in this category are those of the helium dimer ion, He (Ar22 ). It was suggested some time ago that reaction of this species with nitrogen,... 

The accuracy of the MO-VB wavefunction is expected to be close to that of a full SD-CI wavefunction involving excitations to the full virtual spaces of each monomer (vertical excitations). Very recently, a new version of the MO-VB optimization scheme has been developed that is apt to guarantee that the wavefunction approaches as close as possible the full SD-CI limit, via saturation of the optimal virtual space. Explorative calculations on the very challenging helium dimer system are encouraging. 

Fig. 17. The calculated MO-VB interaction potential (JiEh) for the helium dimer system.

Calculations of the third-order exchange contributions have not been performed thus far. The results reported in Ref. (81) for the total third-order exchange effect for the helium dimer suggest that they quench a large part of the third-order polarization contribution. 

A symmetry-adapted perturbation theory approach for the calculation of the Hartree-Fock interaction energies has been proposed by Jeziorska et al.105 for the helium dimer, and generalized to the many-electron case in Ref. (106). The authors of Refs. (105-106) developed a basis-set independent perturbation scheme to solve the Hartree-Fock equations for the dimer, and analyzed the Hartree-Fock interaction energy in terms of contributions related to many-electron SAPT reviewed in Section 7. Specifically, they proposed to replace the Hartree-Fock equations for the... 

Korona T, Williams HL, Bukowski R, Jeziorski B, Szalewicz K (1997) Helium dimer potential from symmetry-adapted perturbation theory calculations using large Gaussian geminal and orbital basis sets. J Chem Phys 106 5109—5122... 

The very first demonstration of molecule interference dates back to the days of Estermann and Stern [Estermann 1930] who demonstrated experimentally diffraction of 11-2 at a LiF crystal surface in 1930. Further experiments with diatomic molecules had to await progress and interest in atom optics. A Ramsey-Borde interferometer was realized for the iodine dimer in 1994 [Borde 1994] and was recently used [Lisdat 2000] with K. Similarly, a Mach-Zehnder interferometer was demonstrated [Chapman 1995 (a)] for Na2. The nearfield analog to the Mach-Zehnder interferometer, a Talbot-Lau interferometer, was recently applied to experiments with L12 [Berman 1997], Diffraction at nanofabricated gratings also turned out to be the most effective way to prove the existence of weakly bound helium dimer [Schollkopf 1996] and to measure its binding energy [Grisenti 2000],... 

The scalar spin-spin coupling has been calculated by Pecul for the helium dimer using full configuration interaction and EOM-CCSD methods. The... 

Hesselmann, A. and Jansen, G. (2003). The helium dimer potential from a combined density functional theory and symmetry-adapted perturbation theory approach using an exact exchange-correlation potential. Phys. Chem. Chem. Phys., 5, 5010-14. 

Total energy (Etotai) and full Cl correlation energy (Ecorr) for the beryllium atom and the helium dimer (Rt = 5.6 bohr) obtained with the numerical molecular basis sets. All energies are in Hartree. 

Recently, the convergence of the many-body sapt expansion for the interaction-induced dipole moment of He-H2 (24) and the polarizability of He2 (24, 32). has been checked by comparison with fci results in the same basis set. The numerical results for the helium dimer are summarized in Table 1 where we consider the anisotropy 7 and the trace a 7 = Aa — Aasx and a = (Act, + 2Aazz)/3, in which the z-axis is the molecular axis. An inspection of Table 1 shows that for all distances considered in Ref. (32) the many-body SAPT expansion reproduces the fci results to... 

Table 2 Basis set dependence of the SRS interaction energy for the ground-state of a helium dimer at R = 5.6 bohr. E(n) and 6(n) are defined as in Table 1. Energies are in /ihartree.

B. Liu and A. D. McLean, ]. Chem. Phys., 91, 2348 (1989). The Interacting Correlated Fragments Model for Weak Interactions, Basis Set Superposition Error, and the Helium Dimer Potential. 

One of the simplest van der Waals complexes is the helium dimer. The small size of the system has made it possible to evaluate the He- He spin-spin coupling constant in an accurate manner, at the full configuration interaction level The Fermi-contact term has been found to have non-negligible value of 1.3 Hz at R=5.6 a.u. (dose to the energy minimum), while the other contributions are practically zero. The coupling decreases very fast, in an exponential manner, with the intemuclear distance R. For R equal to 4 a.u. it is over 22 Hz, while for R over 7 a.u. it falls below... 

By exciting a pair of metastable He atoms in the metastable state 2 5 1 with a narrow-band laser at X = 1083 nm, bound vibrational levels of the helium dimer He in the electronic 0 state (2 S + 2 Pq) could be populated in a magnetic trap [619]. The inner turning point at R = ISOao and the outer turning point at 1150 0 = 57.5 nm show that these excited dimers are indeed giant molecules in terms of their size. 

J. Leonard et al., Giant helium dimers produced by photoassociation of ultracold metastable atoms. Phys. Rev. Lett. 91, 073203 (2003)... 

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